1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus and is suited for an image forming apparatus that uses an electrophotographic process, such as a laser beam printer (LBP), a digital copier, or a multifunction printer.
2. Description of the Related Art
Typical LBPs, digital copiers, and multifunction printers use an optical scanning apparatus.
The optical scanning apparatus periodically deflects a pencil of rays (light beam) being modulated in response to an image signal and emitted from a light emitting unit using a optical deflector including a rotatable polygonal mirror.
The optical scanning apparatus causes a deflected light beam to converge on a surface of a photo conductor in the form of a spot using an imaging optical system having fθ characteristics and records an image by scanning the surface.
There are various optical scanning apparatuses that include an imaging optical system having a single imaging lens with the aim of reducing its size (see, for example, Par. 21 and FIGS. 1 and 16 of Japanese Patent Laid-Open No. 2003-287695, corresponding to 1.66 of col. 5 through 1.16 of col. 6 U.S. Pat. No. 6,847,473)
This patent document discloses an optical scanning apparatus that includes an imaging optical system having a free-form transmitting surface and a free-form reflecting surface, that reflects a light beam passing through the transmitting surface after being emitted from an optical deflector at the reflecting surface, and that causes the reflected light beam to re-pass through the transmitting surface.
The optical scanning apparatus described in the above patent document causes a light beam to obliquely enter the deflecting surface of the optical deflector within a sub-scanning section. Thus, there is a problem in that pitch unevenness occurs in the sub-scanning direction because of a shift decentering error of the deflecting surface.
When projecting a light beam within a sub-scanning section, the optical scanning apparatus described in the above patent document causes the light beam to enter the deflecting surface of the optical deflector from an oblique direction with respect to the normal to the deflecting surface (or with respect to the optical axis of the imaging optical system). Accordingly, it is necessary to enhance machining precision of the optical deflector.
In addition, because the optical scanning apparatus described in the above patent document defines a power of the transmitting surface and a power of the reflecting surface in order to separate a light beam that first passes through the transmitting surface and a light beam that re-passes through the transmitting surface from one another in a sub-scanning section and in order to mitigate curvature of field at the photo conductor in a sub-scanning direction and have a constant magnification in the sub-scanning direction, a problem still remains in that a reduction in thickness of the optical scanning apparatus is not achieved.